The present invention relates to a plasma processing apparatus for generating a plasma by utilizing the energy of a microwave and, more particularly, to an electron cyclotron resonance (ECR) type plasma processing apparatus.
In the manufacturing process of a semiconductor device, a plasma processing apparatus is used for film formation, etching, ashing, and the like. As a plasma processing apparatus, an ECR type apparatus which generates a plasma by utilizing a microwave and a magnetic field generated by an annular coil is known. A plasma processing apparatus of this type can stably generate a plasma even in a high vacuum state having a comparatively low pressure of about 0.1 Torr to several 10 mTorr.
Jpn. Pat. Appln. KOKAI Publication No. 3-17273 discloses an example of an ECR type plasma processing apparatus. In the apparatus of this reference, a means for generating a magnetic field is arranged in a plasma generation chamber, and a waveguide for introducing a microwave is connected to the plasma generation chamber. Electrons are excited by electron cyclotron resonance caused by the cooperation of the microwave and magnetic field, and the process gas is converted into a high-density plasma.
FIG. 16 shows the schematic arrangement of an ECR type plasma processing apparatus. A microwave inlet window 4 is formed in the ceiling portion of a processing chamber 2. A microwave generated by a microwave generator 6 is transmitted to the microwave inlet window 4 through, e.g., a rectangular waveguide 8 and a conical waveguide 10. The microwave introduced from the microwave inlet window 4 into the processing chamber 2 cooperates with a vertical magnetic field generated by a magnet 12 arranged on the outer side of the upper portion of the processing chamber 2 to cause electron cyclotron resonance, thereby converting the process gas into a high-density plasma.
The microwave is transmitted through the waveguide 8 in the TE.sub.10 mode, is converted into the TE.sub.11 mode when entering the conical waveguide 10, and is introduced into the processing chamber 2. When the TE.sub.11 -mode microwave is observed in relation to a target object, e.g., a semiconductor wafer w, its field density is high at the central portion of the wafer, and is gradually decreased toward the peripheral portion of the wafer. Since the progress of plasma processing (e.g., the etching rate or film formation rate) on the wafer is substantially proportional to such a field density distribution, the non-uniformity in field density distribution degrades the planar uniformity of plasma processing. In particular, as the wafer size increases from 8 inches to 12 inches, it becomes more and more difficult to increase the uniformity in plasma density. Therefore, strong demand has arisen for solving the above problem.
Jpn. Pat. Appln. KOKAI No. 2-170530 discloses a technique for solving this problem. According to this reference, a plurality of independent microwave generators are disposed, and microwaves from these microwave generators are separately introduced into the processing chamber. In this case, however, because, e.g., the phases of the microwaves from the respective generators are not aligned with each other, the planar uniformity in plasma density and film thickness cannot be increased very much even though the plurality of generators are disposed. Due to the presence of the plurality of microwave generators, a large increase in apparatus cost cannot be avoided.
Wave Propagation and Plasma Uniformity in an Electron Cyclotron Resonance Plasma Etch Reactor in Jpn. J. Appl. Phys. Vol. 32 (1993) pp. 3007-3012 discloses still another example of an ECR type plasma processing apparatus. In the apparatus disclosed in this reference, after microwaves are passed through two couplers separately disposed, they are combined again. The microwaves are transmitted in the TE.sub.11 mode and the TM.sub.01 mode in the respective couplers. In this case, however, since the couplers, a .lambda./4 polarizer, and the like are used, the structure of the apparatus is rather complicated.